The present invention relates generally to the process of fabricating semiconductor devices, and, more particularly, relates to the process of fabricating CMOS devices.
One prior technique of fabricating MOS devices, for example, used the Planox process. In this process a layer of silicon nitride is deposited on the wafer. The silicon nitride layer is then etched in a desired manner. Next a layer of silicon dioxide (a thick field oxide) is grown in those areas where there is no silicon nitride. As normally occurs during this process, the oxide spreads under the silicon nitride layers and produces what is called a "bird's beak." This bird's beak reduces the effective channel width and further increases parasitic inversion regions. After the field oxide is grown, the silicon nitride mask is removed and a second mask is placed upon the exposed wafer and is used to establish drain and source areas. The doping of these areas can be carried out by ion implantation. After the doping, additional silicon dioxide is grown over the source and drain areas as appropriate. The second mask is removed which was protecting the gate areas and these areas then have a thin layer of oxide grown thereon for a gate. Selective areas of the oxide on the source and drain are removed so that contacts such as aluminum can be placed on these areas and on the gate oxide also.
As noted above one disadvantage is the formation of the bird's beak. As the oxide grows below the silicon surface, it forms along the edges of the source or drain junctions therewith. With positive charge build up in the oxide or near the oxide-silicon interface from either processing parameters or total dose irradiation, a parasitic inversion region forms resulting in drain to source leakage. The amount of leakage and circumstances under which it results can be minimized by either hardening of the field oxide itself to reduce charge build-up or increasing the surface concentration to raise the parasitic threshold. The difficulty with the local oxidation (LOCOS) process is that both oxide hardening techniques and implants require controlling the oxide and silicon surface concentration under and adjacent to the bird's beak encroachment. None of the known oxide hardening techniques can be applied selectively to the oxide which encroaches under the silicon nitride mask. A separate masking operation to define an implant to protect this region has severe layout density impact for small geometries because of the alignment tolerance requirements to allow re-alignment to the oxide edge.